1. Field
This disclosure relates generally to integrated circuits, and more specifically to one-time-programmable nonvolatile memory cells.
2. Related Art
Nonvolatile memory (NVM) is memory that can retain stored information, or data, even when not powered (i.e., when electrical power for the memory circuit has been removed). One type of nonvolatile memory is initially manufactured without stored data, and then later programmed by selectively programming a fuse element in memory cells. The fuse elements are programmed by allowing the fuse to remain intact to represent a first data state, or by “blowing” the fuse to create an open or high impedance circuit to represent a second data state. The programmable fuse can be programmed during manufacturing of the memory, or later, after manufacturing.
Thus, the fuse has two states: normal and blown. In one embodiment, the fuse can be a short piece of polysilicon that is used as a fuse element. To “program” the fuse element, a suitably large current can be passed through the fuse element, which then heats up and changes the resistance (i.e., impedance) of the fuse element.
One characteristic of a good nonvolatile memory that uses fusible elements is accuracy in reading the state of the fuse, which state represents the stored data. When the programming current is applied to the fuse, the resulting impedance of the fuse can fall within a range, as the fuse may or may not be blown completely. In spite of this range of impedances, accuracy in reading the state of the fuse remains paramount.